The present invention relates to radio frequency (RF) coaxial cable, and more particularly to lossy RF coaxial cable having applications over wide frequency ranges.
As is known, in some applications radio frequency energy systems, such as radar systems, require the use of "lossy" RF cabling, that is, cables having a predetermined amount of attenuation in excess of a normal cable loss in order to provide buffering and reduce unwanted signal interactions. In one such application, the lossy cable is utilized as a "buffer" to attenuate and couple an RF signal to an amplifier to ensure the signal applied to the amplifier input is of sufficiently low power so that the amplifier operates in its linear range and does not become saturated. In another application, the lossy cable provides coupling between devices having relatively high voltage standing wave ratios (VSWR) to thereby "pad-down" the high VSWRs and reduce the VSWR product seen by an input signal applied to the assembly. Typically, such applications require the lossy RF cables to have nearly constant loss over a wide operating bandwidth, such as over more than one octave in frequency.
Conventional lossy RF cable assemblies for such applications comprise a coaxial cable in series with a discrete attenuator. The coaxial cable is of coneentional construction, having a center conductor comprising copper or silver-plated wire spaced from a conductive shield by dielectric material. The attenuator is selected to provide the cable assembly with a nominal attenuation (for example -3.0 dB) at the midband operating frequency of the assembly. Over the operating bandwidth, the loss provided by the attenuator remains substantially constant (i.e., flat), however, the cable loss typically changes with variations in frequency. Thus, the total attenuation characteristics of the cable assembly vary somewhat from the nominal attenuation (e.g. -3.0 dB) over the operating frequency bandwidth thereof, for example, by .+-.0.5 dB.
While such lossy cable assemblies are satisfactory in some applications, such cable assemblies are relatively large and bulky due to the presence of the attenuators therein and thus may be unsuitable for use in small RF systems or in RF systems where low weight is required (for example, in airborne applications). Also, such cable assemblies are relatively expensive since an attenuator is used with each RF cable. Further, a plurality of such lossy cable assemblies often have different frequency responses. That is, the attenuation characteristics over the operating frequency bandwidth of one such cable assembly may be substantially different from that provided by another such cable assembly, since the discrete components of such assemblies (e.g. the attenuators) typically have different frequency response characteristics. Thus, one of such lossy cable assemblies is not readily interchangeable with another one of such lossy cable assemblies and system adjustments typically are required when replacing such lossy cable assemblies to compensate for the different frequency responses thereof. Also, due to the presence of the attenuators in such cable assemblies, it is difficult to phase match the electrical lengths of two or more of such cable assemblies to tight tolerance (such as within a few degrees) over a wide frequency band (such as over more than one octave). Additionally, the attenuator increases the voltage standing wave ratio (VSWR) of the lossy coaxial cable assembly, thereby limiting the effectiveness of the lossy cable assembly in the aforementioned "pad-down" application.